Wednesday, October 15, 2014

This is the second year teams from our school have had a go at the competition. We have a keen but young group of pupils and I hope that by exploring how the ciphers could have been solved after the answers have been published, we will have a better prepared team ready to tackle next year's challenges.

Also, I thought this blog would be a good place to consider some of the educational benefits of tackling competitions like this. So here is how I solved Challenge 1A. I'll follow up in the next day or so with a consideration of the computing and maths skills that could be developed through cipher cracking activities.

The first challenge was pretty straightforward. The cipher text given was:

The passage looked like a letter between Harry and Mark (two people named in the introductory text related to the challenge) so an obvious crib presents itself if you guess that QEVO at the start of the cipher text is Mark and that LEVVC at the end is Harry. This suggests the following substitutions:

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

.

.

y

.

a

.

.

.

.

.

.

h

.

.

k

.

m

.

.

.

.

r

.

.

.

.

(Note: from now on I will adopt the convention of showing Cipher text in capital letters and plain text in lowercase, even to the extent of showing proper names like "Harry" as "harry".)

(Note: to make it easier to process in the spreadsheet I used to help me crack this cipher, I removed the extra lines, effectively turning it into a single paragraph, but otherwise left the punctuation and spacing intact.)

You now have a choice to make:

You can look at the partially deciphered text to see if any further substitutions suggest themselves. For example, the three letter grouping XhI appears many times in the text. Since "the" is a very common three letter word, there is a good chance that X=t and I=e. Similarly, the letters Ahy appear three times, which means A is probably w. Substitute these letters and then look at the text again. Keep looking for recognisable words and guessing letters until you have decoded the whole message.

In this case, the easier option is to guess what kind of cipher was used and see if your guess is right by trying it out on the cipher text. Since we have guessed that E=a, we could further guess that Mark and Harry are using a Caesar cipher where plain text letters are moved forward four places to get the cipher text letter. So a goes to E (b to c to d to E - four places). Use your code wheel, set A to E and check if the other letters we have chosen match our guesses. You should see that the guesses match all round the wheel. That is r goes to V (s to t to u to V - four places) and y goes to C (z to a to b to C) etc.

What ever method you choose, you should be able to decipher the rest of the message and read:

mark,
thanks for bringing me in on this one, seems like a fascinating case.
i have three questions:
why would the flag day associates want a ship?
why would they want this ship?
why would they want this ship now?
having read the attached document i suspect that the answers are all
related to the question of what exactly she and her flag day associate
crew were trying to survey.
i am guessing that you already checked out the onboard gps system for
information about her movements, but if you did find anything i would be
fascinated to hear about it. in the meantime i am pretty sure that you
know more about the flag day associates than you have told me, so a
briefing would be much appreciated.
all the best,
harry

So, Challenge 1A solved. Before going on to solve Challenge 1B, the next post will talk about the spreadsheets I developed to help crack the ciphers.